The determination of water by the Karl-Fischer method has been known for a long time (Angew. Chemie 48, 394 (1935)). In carrying out this method, a solvent is generally initially taken and, in order to remove the water content, is titrated to the end point with Karl-Fischer solution. A specific amount of the sample to be examined is then dissolved in the solvent and titrated with the Karl-Fischer solution until the end point is reached. The water content is then calculated from the amount of the sample, the consumption of Karl-Fischer solution and the factor of the solution.
The solvents employed are generally lower, monohydric alcohols, such as methanol and ethanol, or dihydric alcohols which are etherified on one side, such as ethylene glycol monomethyl ether. Examples of other solvents are pyridine, chloroform, dimethyl sulfoxide, formamide, dimethylformamide and other conventional solvents. The solvents are employed either on their own or as mixtures, in order to improve the solution properties for certain groups of substances or to suppress interfering side reactions.
However, the solubility of nonpolar compounds, such as petroleum fractions or diesel oils in these solvents is very poor. The problem is increased by the fact that these substances often contain very little water and it is therefore necessary to dissolve a fairly large amount of sample. Under these conditions, accurate determination of water by the Karl-Fischer method is not possible without the addition of a solubilizer. Previously, pyridine, halogenated hydrocarbons (for example chloroform), aromatic hydrocarbons (for example xylene) or aprotic solvents have been employed for this purpose, but none of these affords satisfactory results.